[+] Fix: Enhance request parsing to avoid hq parsing error

[Sy0307]

Related issue: #518 #511

Issue: xqc_hq_parse_req uses sscanf("%s %s") without bounds, so a long URL from neqo overruns the 256‑byte buffer. Since xqc_hq_request_recv_req didn’t null‑terminate the buffer, later strlen() walks into uninitialized memory, causing an ASan heap-buffer-overflow and leaving HQ requests stuck after “hq recv CR LF”.
Fixes:
Build a bounded format string (e.g. "%15s %255s") before calling sscanf, so both method and path respect the buffer size.
After reading from the QUIC stream, check whether recv_cnt hits the buffer limit; if not, append '\0'. This guarantees strlen()/sscanf() operate on a valid C string.

When neqo client interop with xquic server, xquic server will trigger core dump & timeout, gdb log like this:

=================================================================
==8799==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x615000002080 …
    #3 0x100f51c80 in xqc_hq_parse_req xqc_hq_request.c:234
    #4 0x100f52aac in xqc_hq_request_recv_req xqc_hq_request.c:297
    #5 0x100f58600 in xqc_demo_svr_hq_req_read_notify demo_server.c:632
    

This PR will solve the issue of parsing error if server run as expected.

[Yanmei-Liu]

Solve interop test error. Looks good.